Electron capture dissociation (ECD) and electron transfer dissociation (ETD) have been employed as structural interrogation tools to analyze biomolecules, particularly proteins and peptides. Both dissociation methods have shown extensive cleavage of the peptide back-bone bonds while preserving post-translational modifications (PTMs) arising from, for example, phosphorylation and glycosylation. The major structurally informative dissociation channels in both ECD and ETD often give rise to complementary c- and z-type fragment ions, while conventional ion activation methods, such as collision-induced dissociation or infrared multi-photon dissociation, give b- and y-type fragment ions. The latter dissociation methods often suffer from the difficulty of identifying the site of modification due to the propensity for cleaving PTMs.
Efficient ECD is mainly implemented in one form of mass spectrometry, that is, Fourier transform ion cyclotron resonance mass spectrometry, although some experiments describing the implementation of ECD in electrodynamic ion traps have been reported. ETD resulting from electron transfer via ion/ion reaction, is readily effected in electrodynamic ion traps, including quadrupole 3-D ion traps and linear ion traps (LITs). Due to its greater ion capacity and higher capture efficiency for injected ions, the LIT has advantages over the 3-D ion trap. Several ways of effecting ion/ion electron transfer dissociation reactions within a LIT are known, where both polarity ions can be produced and injected into the LIT from the axial direction, as shown in FIG. 1. One involves the storage of neither ion polarity and relies on reactions taking place between the ions of opposite polarity as they are continuously admitted into the LIT (Method A). The likelihood for ion/ion reactions in this mode is expected to be low as the relative velocities of the ions are high.
Another method (Method B) employs mutual storage of oppositely charged ions, which is expected to provide low velocities. However, this method requires the application of radio frequency (RF) voltages to the containment lenses of the LIT or the application of unbalanced RF voltages to the quadrupole array.
Ion/ion electron transfer dissociation reactions performed in a LIT have employed the mutual storage mode. Previous work demonstrated the use of positive ion transmission/negative ion storage mode for ion/ion proton-transfer reactions in a LIT by using electrospray ionization (ESI) and atmospheric sampling glow discharge ionization (ASGDI) sources. However, no ion/ion electron-transfer reactions appear to have been effected.